This invention relates to a hydrogen absorbing electrode and a metal oxide-hydrogen secondary battery provided with the hydrogen absorbing electrode as a negative electrode.
Recently, a hydrogen absorbing alloy which is capable of reversibly absorbing and desorbing hydrogen for use as an energy source is extensively applied to the negative electrode of a nickel-metal hydride secondary battery.
A rare earth elements-based hydrogen absorbing alloy having an AB.sub.5 type crystalline structure; a titanium-based hydrogen absorbing alloy having an AB type or AB.sub.2 type crystalline structure; and a zirconium-based hydrogen absorbing alloy (Laves phase alloy) having an AB.sub.2 type crystalline structure are all capable of reacting with hydrogen at the room temperature, and relatively excellent in chemical stability so that they are now extensively studied as useful for an electrode material of a secondary battery. In particular, a metal oxide-hydrogen secondary battery, e.g. nickel-hydride secondary battery, which is provided with a hydrogen absorbing electrode (negative electrode) containing an AB.sub.5 type hydrogen absorbing alloy exhibits a discharge capacity of as high as 80% or more of the theoretical discharge capacity, so that any further increase of the discharge capacity may not be expected.
On the other hand, a rare earth element-nickel intermetallic compound takes many other crystal structures in addition to the AB.sub.5 type structure. For example, it is reported that an intermetallic compound containing a larger quantity of a rare earth element than that in the AB.sub.5 type intermetallic compound is capable of absorbing a larger amount of hydrogen as compared with the AB.sub.5 type intermetallic compound at the room temperature (for example, Mat. Res. Bull., 11, (1976) 1241). However, it is also reported that since the aforementioned rare earth element-nickel intermetallic compound tends to retain hydrogen too stably, the hydrogen thus absorbed by the rare earth element-nickel intermetallic compound can be hardly released or desorbed therefrom. For example, it is reported in J. Less-Common Metals, 136 (1987) 121 that when a CeNi.sub.3 --CeCo.sub.3 alloy was allowed to undergo a charge/discharge in a 1M KOH electrolyte, a capacity of about 700Q/g was obtained in the charging process, but a phenomenon of CeNi.sub.3-x Co.sub.x H.sub.3 .fwdarw.CeNi.sub.3-x Co.sub.x H.sub.1 was recognized in the discharging process. Since an electrode containing this kind of rare earth element-nickel intermetallic compound is hardly capable of completely desorbing hydrogen at the occasion of discharging, no one has succeeded up to date to put this electrode into practical use as an electrode of a metal oxide-hydrogen secondary battery.